Try starting your system in Safe Mode and perform a full antivirus/antispyware scan of your system.
Check the Windows "System" and "Application" logs in the Event Viewer to see if there's any record related to this problem.
If this strange behaviour is recent, you could try reverting back your system to a previous well functioning state by applying a restore point after performing the suggested checks.
Bye.
Luigi Bruno - Microsoft Community Contributor 2011 Award